The present invention relates generally to electrical stimulators for biological tissue and more particularly to a linear current output circuit for electrical stimulators of biological tissue.
Electrical stimulators providing an electrical stimulus signal are useful for exciting biological tissue. One significant use for electrical stimulators of this type is for transcutaneous electrical nerve stimulation (TENS) which generate carefully controlled electrical stimulus signals which are delivered via a suitable electrode through a patient's skin to underlying biological tissue. The electrical stimulus signals can be utilized for the purpose of masking pain signals within the nervous system, for example, the sensation of pain felt by a patient after surgery. Because of a patient's response to transcutaneous electrical nerve stimulation may vary significantly, a wide range of electrical stimulus parameters must be provided. A second use of electrical stimulators is for neuromuscular stimulation (NMS) in order to initiate or control muscular action in a patient. Since a wide variety of muscular actions are available, again a wide variety of electrical stimulus signals can be provided.
Recently, electrical stimulators for biological tissue have utilized digital electronics. An example of such a digital electrical stimulator internally generates or provides a digital output word or a series of digital output words indicative of an electrical stimulus signal to be generated. However, while the electrical stimulator generates digital output words indicative of the electrical stimulus signal to be generated, the actual electrical stimulus signal to be output to the biological tissue is analog in nature. Therefore, such a digital electrical stimulator must convert the digital output word, or series of digital output words, to an analog electrical stimulus signal. Thus, such a digital electrical stimulator must provide an output stage in which the digital output word, or series of digital output words, is converted to an analog electrical signal.
While the accuracy of the digital output words can be controlled closely and depends upon the number of bits or digits utilized to represent such digital output word, such accuracy is lost if that accuracy cannot be maintained during the conversion process from the digital output word to the analog electrical stimulus signal.
This need is contrasted to most digital to analog output stages, or circuits, which are concerned with achieving only the highest effeciency possible or in maintaining a constant current output without concern for linearity.